Since heat-generating parts such as power transistors and thyristors degrade their characteristics by the heat generated, a countermeasure is taken in the prior art upon installation by attaching a heat sink to the heat-generating part so that the heat may be dissipated and released to a metallic chassis of the apparatus. In this case, a heat-dissipating insulating sheet of silicone rubber loaded with heat conductive filler is used between the heat-generating part and the heat sink for the purpose of improving electrical insulation and thermal conductivity.
As the material of heat-dissipating insulating sheets, JP-A S47-32400 (Patent Document 1) discloses an insulating composition comprising 100 parts by weight of a synthetic rubber such as silicone rubber and 100 to 800 parts by weight of at least one metal oxide selected from beryllium oxide, aluminum oxide, aluminum oxide hydrate, magnesium oxide, and zinc oxide. As the heat-dissipating material used where insulation is unnecessary, JP-A S56-100849 (Patent Document 2) discloses an addition curable silicone rubber composition loaded with 60 to 500 parts by weight of silica and a heat conductive powder such as silver, gold or silicon. However, these heat conductive materials have a thermal conductivity of up to 1.5 W/m·K. There is left the problem that if the composition is heavily loaded with a heat conductive filler for increasing thermal conductivity, fluidity is reduced and molding or working ability is extremely exacerbated.
As one solution to this problem, JP-A H01-69661 (Patent Document 3) discloses a highly heat conductive rubber/plastic composition comprising alumina in the form of spherical corundum particles of cutting edge-free shape consisting of 10 to 30% by weight of alumina particles with an average particle size of up to 5 μm and the balance of single particles with an average particle size of at least 10 m. Also, JP-A H04-328163 (Patent Document 4) discloses a heat conductive silicone rubber composition comprising 100 parts by weight of a base polymer which is a mixture of a gum-like organopolysiloxane having an average degree of polymerization of 6,000 to 12,000 and an oily organopolysiloxane having an average degree of polymerization of 200 to 2,000 and 500 to 1,200 parts by weight of spherical aluminum oxide particles. On use of either of these compositions, however, merely the combination of particles and the viscosity adjustment of silicone base are insufficient to improve molding or working ability when it is attempted to load 100 parts by weight of the base polymer component with at least 1,000 parts by weight of aluminum oxide powder (i.e., at least 70% by volume of aluminum oxide).
As the means for improving molding or working ability, JP-A 2000-256558 (Patent Document 5) discloses a heat conductive silicone rubber composition containing 0.1 to 50% by volume of a hydrolyzable group-containing methylpolysiloxane as wetter. Although the heat conductive silicone rubber composition is improved in molding or working ability by this means, its adhesion to substrates is still insufficient. Also, JP-A 2008-239719 (Patent Document 6) discloses a silicone elastomer composition capable of forming a silicone elastomer experiencing a reduced hardness change upon heat aging. While an adhesion test to aluminum substrates is described, there is left the problem that its adhesion to organic resins is insufficient. As described above, an attempt to load a heat conductive silicone composition with a large amount of heat conductive filler such as alumina for the purpose of enhancing the thermal conductivity thereof encounters the problem that the cured physical properties are substantially degraded.
Also, power control units mounted on advanced hybrid cars, electric vehicles and fuel cell vehicles need a reactor for applying the battery voltage across the motor after boosting up. In conjunction with the trend toward space saving and size reduction of the power control unit, the reactor as one component of the unit is also required to be compact and its internal structure currently becomes more miniaturized and complicated. Additionally, the reactor interior reaches high temperature. It is urgently desired to develop a heat conductive silicone adhesive composition for use with reactors having a high heat dissipation capability of at least 0.5 W/m·K and experiencing minimized changes of physical properties on heat or moist-heat aging at the same time.
JP-A 2011-122000 (Patent Document 7) discloses a silicone composition as heat conductive potting compound, which maintains good flow and effective potting around reactor components and has good physical properties after curing, even when heavily loaded with a heat conductive filler so that the composition may be highly heat conductive, and a method for selecting a heat conductive potting compound, but refers nowhere to changes of physical properties after heat or moist-heat aging. The composition is still insufficient in this respect.